1. Field of the Invention
The invention relates to a method of forming a semiconductor device, and more particularly, to a method which employs doping with different kind of dopant of different concentration to change electric field of a semiconductor device.
2. Description of the Related Art
In FIG. 1, a cross sectional view of a complementary metal oxide semiconductor (CMOS) is shown. In an N-type semiconductor substrate 100, using dopant diffusion or ion implantation, a P-type region 102 is formed. A PMOS transistor is formed on the N-type semiconductor substrate 100, and an NMOS transistor is formed on the P-type region. The NMOS transistor comprises a gate 104 and a P-type source/drain region 106, and the PMOS transistor comprises a gate 104 and an N-type source/drain region 108. The NMOS transistor is isolated from the PMOS transistor by a field oxide layer 110.
As the devices are designed and developed towards a smaller and smaller dimension, the channel length becomes shorter and shorter. The reduced channel length causes a faster operation speed of a device and other short channel effects. According to the formula "E=V/D", in which E is the electric field, V is the voltage, and d is the channel length, providing a constant voltage, the electric filed is increased abruptly with reducing the channel length. The abrupt increase of electric field accelerates the electron in the channel. As a consequence, the electric breakdown occurs.
In the application of radio frequency (RF), a high power gain is required to improve the frequency response. A direct method to increase the power gain is to increase the transconductance gm. While the transconductance gm is increased, the electric field across the channel is increase. In Reverse, the transconductance gm Is increased by increasing the electric field across the channel. However, considering the limitation due to short channel effect and electric breakdown, it is difficult to obtain a high transconductance g.sub.m.